Within a relatively brief time, semiconductor materials have made possible the creation of a wide range of optical and electronic devices which have had a major role in the shaping of our world. The impact of semiconductor devices has been felt from the battlefield to the playground and from the kitchen to the cosmos. In the earliest stages, semiconductor technology was limited by the use of single crystalline materials. These materials were, of necessity, highly pure and possessed of a morphology having an extremely regular and long-range periodicity. The dual and interdependent constraints of periodicity and stoichiometry restricted the compositional range, and hence physical properties of crystalline semiconductor materials. As a result single crystalline devices were expensive, difficult to fabricate and limited in their properties.
While then conventional wisdom dictated that semiconductor behavior could only be manifested in highly ordered materials, it was recognized by S. R. Ovshinsky that the requirements of periodicity can be overcome and that semiconductor behavior is manifested by various disordered materials. In this regard, see "Reversible Electrical Switching Phenomena and Disordered Structures" by Stanford R. Ovshinsky; Physical Review Letters, vol. 21, No. 20, Nov. 11, 1968, 1450 (C) and "Simple Band Model for Amorphous Semiconducting Alloys" by Morrel H. Cohen, H. Fritzsche and S. R. Ovshinsky; Physical Review Letters, vol. 22, No. 20, May 19, 1969, 1065 (C). Disordered materials are characterized by a lack of long-range periodicity. In disordered semiconductors the constraints of periodicity and stoichiometry are removed and as a result, it is now possible to place atoms in three dimensional configurations previously prohibited by the lattice constants of crystalline materials. Thus, a whole new spectrum of semiconductor materials having novel physical, chemical and electrical properties is made available. By choice of appropriate material compositions, the properties of disordered semiconductors may be custom tailored over a wide range of values. Disordered semiconductors may be deposited by thin film techniques over relatively large areas and at low cost and as a result many types of new semiconductor devices have become commercially feasible. A first group of disordered semiconductors are generally equivalent to their crystalline counterparts while a second group manifest physical properties that cannot be achieved with crystalline materials.
As a result of the foregoing, disordered semiconductor materials have come to be widely accepted and a great number of devices manufactured therefrom are in significant commercial use. For example, large area photovoltaic devices are routinely manufactured from amorphous silicon and germanium-based alloys. Such materials and devices are disclosed, for example, in U.S. Pat. Nos. 4,226,898 and 4,217,374 of Ovshinsky et al. Disordered alloy materials have also been used to fabricate photodetector arrays for use in document scanners, cameras and the like. In this regard see U.S. Pat. No. 4,788,594 of Ovshinsky et al. Disordered semiconductor materials have also been used in devices for the high volume storage of optical and electronic data.
It has been found that properties of semiconductor materials in the disordered state will depend upon their morphology and local chemical order and can be affected by various methods of preparation; for example, non-equilibrium manufacturing techniques can provide a local order and/or morphology different from that achieved with equilibrium techniques; and as a result, can change the physical properties of the material. In most instances, an amorphous semiconductor will have a lower electrical conductivity than the corresponding crystalline material and in many instances, the band gap energy, optical absorption coefficient and activation energy of corresponding amorphous and crystalline materials will differ. For example, it has been found that amorphous silicon materials typically have a band gap of approximately 1.6-1.8 eV while crystalline silicon has a band gap of 1.1 eV. It is also important to note that amorphous silicon materials have a direct band gap while the corresponding crystalline material has an indirect band gap and as a result, the optical absorption of amorphous silicon is significantly higher than that of crystalline silicon at or near the band edge. It should also be noted that the dark electrical conductivity of undoped amorphous silicon is several orders of magnitude lower than that of crystalline silicon. It can thus be seen that the various physical properties of silicon strongly depend upon its morphology and local order. Similar relationships are found in a large number of other semiconductor materials.
The principle of the present invention resides in the ability to control the local order of a semi-conductor material from that corresponding to a completely amorphous phase through various other local organizations including intermediate order to a state where the local order is so repetitively periodic that the material is in the single crystalline state. The most important and interesting area of the present invention is the ability conferred thereby to control the local order of a semi-conductor material to produce a material which has valuable properties of both the amorphous and crystalline states in a single material. For example, according to the principles of the instant invention, a semi-conductor material having an optical absorption corresponding to that of the amorphous state and transport properties corresponding to those of a crystalline semi-conductor material may be prepared. To do so, the K vector selection rules of the crystalline state must be relaxed so as to remove the constraints from the various phonon mediated properties of the material thereby decoupling them from the morphology of the material.
In accord with the principles of the present invention, decoupling is accomplished by configuring the material as to include clusters of atoms, typically between 12 and 50 angstroms in diameter. The clusters or grains have a degree of order which is different from both the crystalline and amorphous forms of the material. The small size and ordering of the clusters allows them to adjust their band structure to thereby relax K vector selection rules. Practically speaking, the present invention makes possible the manifestation of certain desirable electrical and optical properties of both amorphous and crystalline materials in a single material which may be advantageously manufactured utilizing thin film technology and device configurations.
Even if the ordered clusters of the present invention have excellent electrical properties in and of themselves, the ordered clusters are very small and present in very large numbers in the bulk material. This large number of particles presents a very large surface area which can be a significant source of interfacial disorder which can destroy the advantages gained by the quality of the material constituting the bulk of the ordered clusters. A further subject of the present invention is concerned with the elimination with various interface and impurity states which will inhibit the superior properties of the ordered cluster material. In accord with the principles of the present invention, fluorine is employed to further enhance the properties of the ordered clustered material. Through the use of fluorine, a semi-conductor material, for example silicon or germanium, may be prepared having a very low density of defect states in the gap. This material has excellent optical and electrical properties and may be used in its intrinsic form or may efficiently doped.
Fluorine plays several roles in the preparation of the ordered cluster semi-conductor material of the present invention. Fluorine interacts with the depositing material to permit the nucleation and growth of clusters having the proper size, order and composition. Fluorine cleans the surface of the clusters as they are being deposited by removing weak or otherwise deviant bonded material and by scrubbing away impurities. Fluorine also acts in the bulk of the semi-conductor material constituting the ordered clusters to moderate the bulk electrical properties of the material.
Through the use of the present invention, it is possible to manufacture novel, semi-conductor materials having unique and superior combinations of properties heretofore unavailable in a single material. Through the use of the present invention, an indirect band gap material, such as crystalline silicon may be configured into an ordered cluster material causing it to be an approximate direct band gap material at 1.2 eV. This presents a significant improvement in a variety of electrical devices such as photovoltaic devices. The approximate direct gap ordered cluster silicon material has an optical absorption coefficient which is approximately two-to-four orders of magnitude higher than that of single crystal silicon at 1.2 to 1.5 eV. This allows for the manufacture of photovoltaic devices which are very thin since no more than one micron of semi-conductor material is needed to accomplish the same absorption of light as several hundred microns or more of single crystalline material. In addition to the high absorption, the ordered cluster silicon material retains the low band gap energy and good electrical transport properties of single crystalline silicon.
Fluorine plays several important roles in the deposition of the ordered clustered material of the present invention, in the plasma or vapor state as well as on the surface of the cluster, in the bulk of the clusters and at the interfaces between the clusters. It also is beneficial in reducing the density of defect states in the gap of the material. Fluorine is a very active etchant material and it is generally preferable that it be moderated, for example by dilution with hydrogen.
The present invention resides in, and provides for the ability to decouple and control independently important optical and electrical properties of semi-conductor materials. The present invention improves the performance of any present semi-conductor device and makes possible the manufacture of a variety of novel devices. Accordingly, it will be appreciated that the invention opens a new field of solid state physics in which various heretofore linked properties of semi-conductor materials may be independently and accurately controlled.
The various properties of amorphous and crystalline silicon confer different advantages in various devices. The high mobility of carriers in crystalline silicon is important in high speed semiconductor circuits while the high level of optical absorption of amorphous silicon is ideal for photovoltaic devices since complete light absorption may be accomplished by relatively thin layers of material, making for a lightweight, low cost device. In some instances, one property of a given morphology and local order of semiconductor may be ideal for a particular purpose whereas the value of another property of that same material may not be so well suited. For example, the aforenoted high optical absorption of amorphous silicon is ideal for a photovoltaic device; however, the fairly wide band gap of amorphous silicon does not permit it to address the longer wavelength portions of the solar spectrum. The use of narrower band gap crystalline material in photovoltaic devices increases the portion of the useable spectrum which is addressed and the high conductivity, high mobility and long minority carrier diffusion length in crystalline silicon decreases the series resistance of the photovoltaic device, thereby increasing its overall efficiency; but, the trade-off is that crystalline cells are relatively thick because of their low absorption and hence they are fragile, bulky and expensive.
In accord with the present invention, it has been found that various physical properties of semiconductor materials are decoupled from morphology and local order when those materials are comprised of ordered clusters. As used herein, a semiconductor having ordered clusters shall be defined as a material having a plurality of atomic aggregations of very short range periodicity, and comprised of a plurality of highly ordered, relatively small atomic aggregations, typically extending no more than 50 atomic diameters. The exact dimensions of the ordered clusters in materials of this type will depend upon the particular semiconductor material in question, but typically are they are in the range of 10 to 50 angstroms and preferably about 12 to 30 angstroms in diameter. The ordered clusters have a periodicity and local order differing from an amorphous or fully crystalline material. Heretofore, a number of materials denominated "microcrystalline" have been prepared. These materials typically include crystallites in the range of approximately one hundred angstroms. Such prior art microcrystalline materials do not manifest any decoupling of properties; that is to say, their physical properties cannot be independently controlled. Thus, it will be seen that such prior art materials one differentiated morphologically and phenomenologically from the ordered cluster material of the present invention.
In the ordered clusters, local order is propagated but does not reach the point of becoming long range order; and therefore, lattice constants of the crystalline state do not become the determinative factor of the material's properties. In the ordered clusters, the bond lengths and angles are much more flexible than in materials with long range periodicity. It has been found that in the ordered cluster state of the present invention, materials exhibit a decoupling of their physical properties from morphology and hence they can manifest combinations of properties not present in the same material when it is in a crystalline, polycrystalline, microcrystalline or amorphous state.
This finding is non-obvious since in heretofore employed crystalline, polycrystalline, microcrystalline and amorphous materials, properties such as band gap and conductivity or optical absorption coefficient have been linked. The important finding of the present invention is that once the ordered cluster state is entered, properties can be independently controlled and it is possible to achieve, for example, a silicon material having a narrow band gap and high conductivity corresponding to the crystalline state and a large optical absorption corresponding to the direct gap of the amorphous state. Clearly, this independent control of individual material parameters achieved through the use of ordered cluster materials makes possible a whole new range of novel semiconductor materials and improved devices which heretofore were unavailable.